Closed drift ion sources have been known since Russian ion thrusters for satellite propulsion were reported in the 1960's. These prior art devices suffer from problems of sputter erosion of the closed drift side walls, loss of energetic electrons to the side walls, and poor beam collimation out of the source.
Side wall erosion has deleterious effects on ion source performance including:                The source wall inserts, magnetic poles, or other plasma exposed surfaces must be routinely replaced. Where replacement is not possible in space thruster applications, wall erosion is eventually catastrophic. In these applications, thrusters are rated in thousands of hours of life with some 2,000–10,000 hours being the published life expectancies.        Ion sputtering of the side walls contaminates industrial ion source processes with the sputtered atoms. In many applications, this precludes these ion sources as potential process tools.        Sputtering of the side walls raises the source wall temperature. This can be a severe problem in space based applications where heat must be dissipated by radiation. The high temperatures experienced by the side walls requires special, expensive materials.        Ions striking the side walls do not exit the source, reducing source efficiency. (Efficiency is the ion current and energy relative to the power supply discharge current and voltage.)        In ion sources operated in the diffuse mode, erosion is particularly problematic if not ruinous. In the diffuse mode, the source is operated at sufficiently high pressure and power to create a neutral, conductive plasma in the gap between the poles. Operating in this mode, the plasma density is dramatically increased, and the electric fields change significantly, increasing ion bombardment of the pole pieces or side walls.        
Other problems generally recognized with prior art ion sources include:                Loss of high energy electrons to the side walls. This especially affects extended acceleration channel type ion sources. Side wall losses of electrons capable of ionizing the propellant gas results in loss of efficiency and side wall heating        Beam spreading outside the source. Here, the ion beam produced leaves the source in a spread cosine distribution rather than the preferred collimated output.        
There are two basic types of closed drift ion sources for which many variations have been offered. The two types are anode layer and extended acceleration channel. Prior art examples for each type of source are described below.
FIG. 1 is a section view of prior art linear anode layer type ion source 100. Additional description of this prior art device can be found in Capps, Nathan, et al., Advanced Energy Industries, Inc. Application note: Ion Source Applications: Si Doped DLC, and in Advanced Energy Industries, Inc. Application note: Industrial ion sources and their application for DLC coating, which are hereby incorporated by reference.
Such a prior art source 100 can either be annular or stretched out to lengths beyond three meters, the confined Hall current design enables extendibility similar to a planar magnetron. FIG. 1 shows the magnetic field lines as calculated and mapped by a two-dimensional magnetic field software program. The field in the gap 120 is created by back shunt 110, permanent magnet 130, and pole pieces 140 and 150. Electrically, poles 140, 150 and shunt 110 are connected to ground, and anode 102 is connected to the positive terminal of a high voltage power supply.
As those skilled in the art will appreciate, the anode 102 in a closed drift ion source is disposed a distance from the gap 120 between the poles 140 and 150, where that distance exceeds the Larmor radius of the captured electrons. As those skilled in the art will further appreciate, the width of the gap 120 is adjusted to maintain a magnetic field of sufficient strength to magnetize electrons and to allow a plasma to exist therein.
Referring to FIGS. 1 and 1A, in prior art device 100, the half bevel shaped poles 140 and 150 produce a magnetic fields with the strongest magnetic field line, described herein as the “primary field line,” emanating from the flat, gap facing pole surfaces 142 and 152. The magnetic configuration and pole shapes of this prior art device, calculated using a Ceramic 8 ferrite type magnet 130, results in a primary field line 170 having a magnetic field strength of 682 Gauss at first end 172 on surface 152, 542 Gauss at second end 176 on surface 142 of outer pole 140, and a minimum strength of 445 Gauss at location 174. As those skilled in the art will appreciate, use of other magnetic materials will change the relative strengths of the field lines but will not substantially change the relative location of the primary line or ratio between surface and gap fields.
By “primary field line,” Applicant means the field line having the least curvature and the strongest field strength in the gap. As the bloom of the field in the gap is viewed, the primary field line is the centerline of the bloom. Field lines to both sides of the primary field line are concave, i.e. curved, and face this field line.
As the magnetic field lines leave the high permeability poles 140 and 150, enter the “air” gap 120, and travel toward the center of the gap, the magnetic field strength lessens. Visually, this is seen as field lines spreading out in the gap. The result of this effect is a magnetic mirror. By “magnetic mirror,” Applicant means the “reflection” of electrons as an electron moves from a region of weaker field to a stronger field.
Applicant has discovered that the mirror ratio is an important aspect of closed drift ion source magnetic design. By “mirror ratio,” Applicant means the ratio of the strong field strength at an end of the field line to the minimum field strength along that field line. For example, in source 100, using calculated field strengths of the primary field line 170 from first end 176 to location 174, the magnetic mirror ratio is 1.22. From second end 172 to location 174 the magnetic mirror ratio is 1.53. Therefore, the minimum mirror ratio for source 100 is 1.22.
In addition, the ratio of the magnetic strengths at the end of the primary field line indicates whether that primary field line is substantially symmetric or asymmetric. By “substantially symmetric,” Applicant means an end-to-end ratio of magnetic strengths of between about 0.94 to about 1.06. For prior art device 100, the ratio of the magnetic field strengths at locations 172 and 176 is about 1.26 indicating an asymmetric mirror field existing between the pole portions.
Applicant has found that a minimum mirror ratio greater than 2 in combination with an end to end ratio of between 0.94 and 1.06 to be optimal. The magnetic pole design of device 100, however, produces weak magnetic mirror fields in gap 120. The result is that when a plasma is disposed in gap area 120, electrons are not strongly focused into the center of the gap. This results in substantial sputtering of the poles 140 and 150 and lower source efficiency.
Pole sputtering is exaggerated when the source is operated in the diffuse mode. This mode is entered when the plasma is dense enough to become electrically neutral. When this occurs, the electric fields change from a gradient field from the cathode poles 140 and 150 in gap 120 to anode 102 to a field dropping from the cathode poles across the dark space to the plasma and from the plasma to the anode. The diffuse mode is entered when a combination of higher process gas pressure and high discharge power produces a bright glow in the gap region. The diffuse mode is visually quite different from the collimated mode making the modes easy to distinguish by eye. In the diffuse mode, sputtering of the poles is increased due to the higher concentration of ions in the gap and the large voltage drop between the plasma and cathode pole surfaces.
Sputtering of the poles contaminates the substrate with sputtered material, causes wear of the cathode poles requiring their regular replacement, adds appreciably to the heat load the source must handle, and makes the source less energy efficient.
In contrast to this prior art device, Applicant's device creates a strong magnetic mirror field in the gap along the primary field line. Such a strong magnetic mirror has dramatic benefits for source operation. Without this focusing mirror field, not only are the poles eroded more rapidly, but the lack of the mirror field focusing effect causes the ion source to produce a broader, less collimated beam.
In addition, prior art device 100 includes a single central magnet. The resulting magnetic field is not symmetrical across gap 120 with one magnetic mirror being stronger than the other. As will be described below, symmetrical magnetic mirrors can be created with strong mirror fields along the central field line to focus the plasma in the center of the gap and optimize magnetic mirror repulsion from the poles.
FIGS. 2 and 2A show a section view of prior art anode layer ion source 200. Device 200 includes shunt 210, magnet 230, poles 240 and 250, and anode 202. An analysis of this pole design shows that the primary field line emanates from the flat faces 242 and 252 of poles 240 and 250, respectively, rather than from the pointed portions 241/251.
Magnetic field line 270 comprises the primary field line in this prior art embodiment. Field line 270 has a magnetic field strength of 683 Gauss at first end 272 on surface 252, 580 Gauss at location 276 on second end 242, and 373 Gauss at location 274 on field line 270. Location 274 comprises the portion of field line 270 having the minimum magnetic field strength. Dividing the magnetic field strength at end 272 by the magnetic field strength at location 274 gives a mirror ratio of 1.83. The magnetic mirror formed between 276 and 274 is 1.55. Therefore the minimum mirror ratio is 1.55. Dividing the strength at end 272 by the strength at end 276 gives a ratio of about 1.17 thereby indicating an asymmetric mirror field existing between the pole elements.
FIGS. 3 and 3A show prior art anode layer source 300 as depicted in FIG. 3 in the publication ‘High Current Density Anode Layer Ion Sources’ by J. Keem, Society of Vacuum Coaters 44th Annual Technical Conference Proceedings. Device 300 includes permanent magnets 331 and 332, in combination with pole portions 340 and 350, and anode 302. Field line 370 comprises the primary field line produced by device 300. Field line 370 has a magnetic field strength of 1013 Gauss at first end 372 on surface 352, 954 Gauss at second end 376 on surface 362, and a minimum strength of 565 Gauss at location 374 on field line 370. Therefore, the minimum mirror ratio for the primary field line for device 300 is 1.69.
FIG. 4A shows a second type of ion source sometimes referred to as an extended acceleration channel type. Extended acceleration channel type ion source 400 is typical of prior art ion thruster propulsion devices. U.S. Pat. No. 5,892,329, in the name of Arkhipov et al., and U.S. Pat. No. 5,945,781, in the name of Valentian, describe such sources. Extended acceleration channel sources are commonly used in space thruster applications but can be adapted for industrial use also.
FIG. 4A shows the magnetic field lines produced by extended acceleration channel source 400. In this source, magnetic poles 440 and 450 are electrically floating. An electron source 480 serves as the cathode with anode 402 located inside ceramic isolator 490. Anode 402 is positioned at the bottom of channel 422 such that electrons must pass through magnetic fields crossing gap 420 to reach anode 402.
It is known that the ceramic side walls of an extended acceleration channel source, such as source 400, tend to be eroded by ion bombardment. Because prior art device 400 separates the magnetic poles 440 and 450 from the channel with the insulating ceramic 490, and because device 400 does not optimize the pole shapes, a strong magnetic focusing mirror radial field is not created in the channel.
Prior art device 400 produces a primary field line 470 having a magnetic field strength of 1011 Gauss at 472 on the inner surface of insulator 490, 883 Gauss at 476 on inner surface of insulator 490, and a minimum magnetic field strength of 687 Gauss at location 474. This being the case, the minimum magnetic mirror ratio along the primary field line for device 400 is 1.29. The result of a weak mirror field is:                Electrons, accelerated into the magnetic field in the channel by the electric field, are trapped by the magnetic field. Without a containing radial magnetic mirror field, these energetic electrons move along the field lines and can be absorbed by the side walls. Loss of high energy electrons to the walls lowers source ionization efficiency and heats the side walls.        Ambipolar diffusion causes the side walls to be charged negatively, and ions are attracted to the side walls.        The lack of radial electron focusing results in electron distribution across the full channel width. Ions then are created across the full width producing a wider, less collimated beam and added likelihood of ions hitting the side wall.        Only the ions created in the center of the channel experience the electric field pushing them perpendicularly out of the source. Without strong electron focusing, fewer are created in the center.        
FIG. 4B is a section view of ion source 900 described in U.S. Pat. No. 5,763,989 in the name of Kaufmann. Ion source 900 includes poles 940 and 950, in combination with anode 902, in further combination with a magnetic screen shunt similar to that taught in U.S. Pat. No. 5,892,329 in the name of Arkhipov, except the Kaufman shunt is arranged to allow a single permanent magnet to be used. This shunt technique produces a limited focusing effect in the acceleration channel that potentially results in reduced wall losses and less wall erosion.
While producing a mirror field at one side of the gap, the flat pole faces produce a weak mirror field in the center of the gap. Device 900 produces a primary field line having a magnetic strength of 600 Gauss at first end 972, 550 Gauss at second end 976, and a minimum magnetic field strength of 400 Gauss at location 974. Therefore, the minimum mirror ratio for device 900 along the central primary field line 970 is 1.4.
U.S. Pat. No. 4,277,304 in the name of Horiike et al. teaches an ion source and ion etching process. Horiike et al. teach an arrangement for what is termed a grid-less ion source. The ion beam is created by two cathode surfaces with a magnetic field passing between the two surfaces The cathode surfaces and magnetic field are shaped into a racetrack to provide an endless Hall current confinement zone. An anode is disposed on one side of the racetrack magnetic field loop. This arrangement produces an ejection of ions from the side opposite the anode. Other prior art devices implemented electromagnets to create the magnetic field between the cathode surfaces. Horiike et al. teach the use of permanent magnets and a flat facing pole shape.
U.S. Pat. No. 5,359,258 to Arkhipov et al. teaches a closed drift ion accelerator wherein side wall erosion is reportedly lessened by lowering the amount of magnetic field in the acceleration channel by shunting the field with permeable screens. The idea is to move the containment of electrons from the central channel area out closer to the opening. The screens also shape the magnetic field to provide an amount of focusing of the plasma that helps to reduce side wall erosion. According to Arkhipov et al., the focusing effect allows making the channel walls thicker so the source lasts longer too.
Arkhipov et al. nowhere teaches shaping the magnetic poles to produce a strong radial mirror magnetic field in the gap and, more particularly, to produce that strong mirror field along the primary field line. As shown in FIG. 4A, when the poles are separated from the channel by an insulator, the mirror ratio along the primary field line is less than 2.
U.S. Pat. No. 5,838,120 in the name of Semenkin et al. describes an anode layer source comprising a magnetically permeable anode to shape the magnetic field. The use of a magnetic shunt to remove radial, poorly mirrored magnetic field from the central channel, and moving the anode closer to the exit end, may reduce wall erosion. This prior art device, however, only provides marginal improvements. Semenkin et al. nowhere teaches shaping of the magnetic field to produce a strong, focusing mirror field along the primary field line. The device taught by Semenkin et al. results in electrons that are largely free to move along magnetic field lines and, in this case, recombine at the walls.
U.S. Pat. No. 6,215,124 in the name of King discloses a multistage ion accelerator with closed electron drift. In this device, the life and efficiency of the thruster is improved by shunting the magnetic field away from the central accelerator channel region and moving the Bmax field line toward the open end. When this is done, the region of wall erosion moves farther toward the opening, extending the life of the thruster. While use of thin pole pieces could generate a mirror field of some strength, the poles are distanced from the channel by inserts. The result is a weak magnetic mirror field at the exit end with the accompanying negative results.